Occurrence and petrogenesis of diverse S-type granites in an extensional tectonic setting: a case study from the Wongwibinda Complex, eastern Australia

The high-temperature–low-pressure (HTLP) Wongwibinda Metamorphic Complex (WMC) hosts four distinct types of early Permian S-type granitoids belonging, or related, to the Hillgrove Supersuite. We use field relationships, age data and petrogenetic modelling to determine the petrogenesis of these granitoid types. Biotite-only bearing granitoids are the most voluminous (∼50% of the WMC area). A newly characterised second type (<1% of the WMC), here defined as the Wongwibinda monzogranite, has nearly equal proportions of muscovite and biotite, and a distinct chemical signature. The third and fourth types (<1% of the WMC) include pegmatite dykes and a garnet-bearing leucogranite. THERMOCALC modelling of water-fluxed and dehydration partial melting of local metasedimentary rock compositions indicates that the biotite-only granitoids are consistent with crustal melts produced by 30–40% dehydration partial melting of accretionary complex metasedimentary rocks at 6–9 kbar, with an admixture of ∼30% mafic magma. In contrast, the Wongwibinda monzogranite is a nearly pure S-type granite with its composition influenced by assimilated migmatite xenoliths. One viable model for its geochemistry involves a mix of 45% water-fluxed 5–10% partial melt of metasedimentary rocks at 3–6 kbar, 65% assimilated local migmatites and inclusion of 10% residual biotite following the extraction of 20% haplogranite melt, although alternative scenarios are plausible. The leucogranite can be modelled through both dehydration and water-fluxed melting of metasedimentary rocks at 3–6 kbar, while the pegmatite composition is consistent with 5% water-fluxed or dehydration partial melts at 3 kbar. It is plausible that the pegmatites, leucogranite and Wongwibinda monzogranite formed coevally with peak metamorphism in the complex. In contrast, the emplacement of the biotite-only Hillgrove Supersuite plutons occurred in the later stages of thermal perturbation, as melts from the deep crust ascended via the Wongwibinda Shear Zone. Small volumes of two-mica Wongwibinda monzogranite type granitoids are commonly associated with HTLP metamorphism in extensional tectonic settings.